Hierarchical Nickel Sulfide Nanosheets Directly Grown on Ni Foam: a

Hierarchical Nickel Sulfide Nanosheets Directly Grown on Ni Foam: 1 a Stable and Efficient Electrocatalyst for Water Reduction and. 2. Oxidation in Al...
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Research Article pubs.acs.org/journal/ascecg

Hierarchical Nickel Sulfide Nanosheets Directly Grown on Ni Foam: A Stable and Efficient Electrocatalyst for Water Reduction and Oxidation in Alkaline Medium Jin-Tao Ren†,‡ and Zhong-Yong Yuan*,†,‡ †

National Institute for Advanced Materials, School of Materials Science and Engineering and ‡Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, China S Supporting Information *

ABSTRACT: Developing high-performance nonprecious electrocatalysts for water reduction and oxidation is highly desirable for future energy supplement. Herein, a facile one-pot strategy is reported to obtain Ni3S4 nanosheets directly grown on Ni foam (NiS/NF) as bifunctional nonprecious electrocatalyst toward full water splitting through a facile hydrothermal-sulfurization method in Na2S solution. The resultant unique structure with integrated hierarchical three-dimensional (3D) configuration can enhance mass transport and charge mobility and facilitate the diffusion of generated gases (H2 and O2). Thus, the prepared NiS/NF exhibits remarkable catalytic activity and outstanding stability for water oxidation and reduction reaction in alkaline electrolyte. For hydrogen evolution reaction (HER), it only needs a low overpotential of −122 mV to render a current density of 10 mA cm−2 with a small Tafel slope of 69 mV dec−1, whereas it delivers a current density of 20 mA cm−2 for oxygen evolution reaction (OER) at the overpotential of 320 mV together with Tafel slope of 71 mV dec−1. Importantly, when NiS/NF is assembled as an alkaline electrolyzor, it only needs a cell voltage of 1.61 V to provide current density of 10 mA cm−2 and maintains this current density for over 20 h with a recession of 4.5%. Various characterizations and controlled experiments reveal that the outstanding activity and robust stability of NiS/NF for electrocatatlytic water splitting are attributed to its integrated electrode configurations of the electrochemically active constituents and the conductive Ni foam and unique superstructure with high electrochemical surface area. KEYWORDS: Water splitting, Oxygen evolution, Hydrogen evolution, Nickel sulfide, Electrocatalyst

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INTRODUCTION The huge depletion of fossil energy and the resulted environmental concerns harvest intense research interest to explore various renewable and sustainable energies to replace traditional fossil fuels for the energy conversion and storage.1,2 Recently, electrolytic water splitting with sustainable energy input, such as renewable solar and wind energy, for the production of hydrogen (H2) offers a promising strategy for this purpose. Efficient water splitting needs active electrocatalysts to boost sluggish electrochemical process of water reduction and evolution. Currently, the state-of-the-art catalysts toward oxygen evolution reaction (OER) and hydrogen reduction reaction (HER) are IrO2/RuO2 and Pt, respectively. However, the unfavorable price and scarcity of them significantly limit their scale-up actual applications.3,4 In contrast, using bifunctional electrocatalysts under the same electrolyte to realize full water splitting has superior merits to simplify the electrolyzer system and consequent facilitate the industrial application,5−8 while the ineffective integration of activity and stability in one same electrolyte then result in inferior electrochemical performance.9,10 Therefore, searching © 2017 American Chemical Society

low-cost, earth-abundant alternative nonprecious electrocatalysts with effective activity and stability toward full water splitting is necessary but difficult. To date, various nonprecious metal alternative catalysts with comparable electrocatalytic activity have been synthesized and explored for water splitting,1,11−15 such as metal phosphides,7,2,16,17 sulfides,18−20 and oxides/hydroxides.21−24 However, one of the widespread drawbacks of most transition metalbased catalysts is their poor electrical conductivity and low specific surface area.25 Some strategies have then been proposed.10,26−30 Combining catalysts to conductive carbon materials or directly depositing catalysts on metal substrates are accessible approaches to resolve those problems;,31,32 though to the powdery catalysts, especially for carbon-based materials, the postcoating strategy would not only increase the preparation cost but also interfere in the conductivity of as-prepared electrodes.6,10,33,34 Thus, using the materials with intrinsic Received: May 6, 2017 Revised: June 17, 2017 Published: July 13, 2017 7203

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of 50 μL of 5 wt % Nafion solution with strong sonication to form homogeneous mixture. Finally, the obtained mixture was carefully dispersed on pristine Ni foam with the same loading mass of NiS/NF and dried in air to evaporate solvent. Preparation of IrO2 on Ni Foam (IrO2). Preparation of IrO2/C colloidal was based on the previous reported method with a little modification.28 First, 0.1 g of K2IrCl6 was dispersed into water (50 mL) containing 0.17 g of disodium hydrogen citrate sesquihydrate under magnetic stirring and pH adjusted to 7.5 with NaOH aqueous solution. The obtained solution was heated at 95 °C under continuous stirring. After naturally cooling down to room temperature, the pH was again adjusted to 7.5 with NaOH and once again heated to 95 °C. Repeating this process until the pH value kept at 7.5. The obtained solution was transferred to a round-bottomed flask with a reflux condenser. The solution was heated at 95 °C for 2 h with bubbled O2, then further dried at 60 °C overnight in vacuum. The final as-obtained solid was heated at 300 °C in air to remove the organic compounds and washed with ethanol. Then, the material was centrifuged and kept at 60 °C. Finally, the as-obtained IrO2 and 50 μL of 5 wt % Nafion solution were dispersed in water−ethanol solution (1 mL, v/v = 1:4) and sonicated for 30 min to get a homogeneous ink. The homogeneous ink was coated on a piece of clean Ni foam with the same load mass of Ni3S4 on Ni foam and dried at room temperature overnight in air. Preparation of Pt/C on Pristine Ni Foam (Pt/C). For comparison, the Pt/C loaded electrode was prepared. First, 20 mg of Pt/C and 10 μL of 5 wt % Nafion solution were dispersed into water and ethanol solution (1 mL, v/v = 1:1) under gentle stirring with sonicated to obtain a homogeneous ink. The homogeneous ink was coated on Ni foam with the same loading with Ni3S4 on Ni foam, followed by drying at room temperature overnight in air to evaporate solvent. Physicochemical Characterization. Scanning electron microscopy (SEM) was carried out via Jeol JSF-7500L microscope at 5.0 keV. Transmission electron microscopy (TEM) was performaned on a Jeol JEM-2100F at an acceleration voltage of 200 kV. The Ni3S 4 nanosheets was scrated off from the NiS/NF and dispersed into ethanol with ultrasonic, and then supported on a copper grid for TEM observation. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max-2500 X-ray diffractionmeter with Cu Kα radiation (λ = 1.5418 Å) operated at 40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS) was done on a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). All XPS spectra were recorded by using an aperture slot of 300 μm × 700 μm; survey spectra were recorded with a pass energy of 160 eV and high-resolution spectra with a pass energy. Electrochemical Testing. For electrochemical tests, commonly the synthesized NiS/NF was directly employed as the working electrode on a WaveDriver 20 Bipotentiostat/Galvanostat (Pine Research Instrumentation, USA) electrochemical workstation in a three-electrode glass cell, with Pt wire and Ag/AgCl used as counter electrode and reference electrode, wherein KOH aqueous solution (1.0 M) was used as the electrolyte and bubbled with pure O2 or H2 before the measurements to reach O2 or H2 saturation. Prior to recording the polarization curves, the working electrodes were scanned for several cycles until stabilization, and a slow sweep scan rate of 1 mV s−1 was applied. The current density was normalized to the geometric surface area. The electrode durability was measured by chronoamperometry measurments at a constant potential value (vs reversible hydrogen electrode (RHE)) that can obtain different anodic/cathodic current. Cyclic voltammograms (CVs) were also recorded over 2000 cycles (50 mV cm−2). The iR compensation toward the polarization curves was corrected. The electrochemical double layer capacitor (Cdl) of the asprepared electrodes was obtained from double-layer charging curves using CVs in a narrow potential region, and the slope of the plot of the current density (certain potential) against scan rate was the double layer capacitance (Cdl). Electrochemical impedance spectroscopy (EIS) was measured by performing an alternating current (ac) voltage with 5 mV amplitude in potentiostatic mode at an overpotential mode

metallic and conductive property as the support to fabricate directly binding-free electrocatalysts is a reasonable route to further boost electrocatalytic performance of transition metal catalysts.3,35−37 However, to further promote the catalytic reaction kinetics and improve the energy efficiency toward overall water splitting, an optimal electrocatalyst structure with rationally designed reaction interface is essential. The previous synthesized nanostructure materials, such as CoP nanorods,10 Co3O4−C nanowires,28 Ni3S2 nanorods,38 and MoP2 nanosheets,39 have been proved the superiority of well-designed configuration. And previous reported nanostructured electrodes could be fabricated by hydrothermal, electrodeposition, and chemical vapor deposition methods, in spite of the relatively complex and tremendous steps for the synthesis of advanced electrode structure, including presynthesis of morphologytailored precursor on various types of substrates and subsequent phosphorization/selenization/sulfurization.5,33,40,41 The whole process is laborious from the viewpoint of practical application and produces poisonous gas. Therefore, it is imperative for large-scale commercial production of nanostructured materials with high performance but facile and lowcost synthesis process. Toward the above-mentioned goals, herein 3D hierarchical porous Ni3S4 nanosheets anchored on Ni foam (NiS/NF) with well-structured configuration as bifunctional electrocatalysts were readily prepared through a one-pot hydrothermalsulfurization strategy of metallic Ni foam in Na2S solution. With the well-designed superstructure, when directly used for water reduction and evolution, the NiS/NF needs an overpotential of −122 mV to provide the current density of 10 mA cm−2 toward HER and 320 mV for OER to deliver 20 mA cm−2. Moreover, assembling an alkaline full water electrolyzer by utilizing NiS/NF as both anode and cathode, the current density of 10 and 20 mA cm−2 can be obtained by this couple at cell voltages of 1.61 and 1.67 V, respectively, together with high stability. The excellent catalytic performance of NiS/NF benefited from the collective effects of the innate catalytic ability of Ni3S4, in situ grown Ni3S4 nanosheets, as well as high conductivity and porous framework of Ni foam as support. Mostly, this synthesis strategy can be potentially applied to obtain morphology-tailored metal sulfides on different metallic collectors.

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EXPERIMENTAL SECTION

Fabrication of Ni3S4 Nanosheet on Ni Foam (NiS/NF). The Ni foam was successively immersed into acetone, ethanol, and deionized water with ultrasonic treatment, and then dried at 80 °C overnight. In a typical run, 30 mL deionized water contains certain molar Na2S was transferred into a Teflon-lined stainless-steel autoclave, and a piece of cleaned Ni foam (1 × 3 cm) was placed into. Next, the autoclave was sealed and kept at 170 °C for 10 h. Finally, until cooling down to room temperature, the Ni foam was taken out and washed with deionized water repeatedly, then dried at 60 °C for 12 h; the resutl was denoted as NiS/NF. The loading mass of Ni3S4 on Ni foam can be calculated by following equation:

M Ni3S4 = w(M Ni3S4 /4MS)

(1)

where w (mg) is the weight increment of Ni foam and M is the molecular or atomic weight. The loading mass of Ni3S4 was about 1.2 mg cm−2. Scratched Ni3S4 Nanosheets Coated on Pristine Ni Foam (scratched NiS). Certain mass of as-prepared Ni3S4 nanosheets scratched off from NiF/NF was dispersed into mixture solution (1 mL) containing water and ethanol (v/v = 1:4), following the addition 7204

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ACS Sustainable Chemistry & Engineering from 0.01 to 100k Hz on a Zaher IM6eX (Zahner, Germany) electrochemical workstation. The measured potentials versus Ag/AgCl were converted to RHE scale according to the Nernst equation:

E RHE = EAg/Ag/Cl + 0.059pH + 0.205

(2)

The Tafel slope was obtained by LSV curves and calculated according to the following equation:

η = a + b log J

(3)

where η is the overpotential, J is the current density, and b is the Tafel slope. The overpotential was calculated as follows: η = E RHE − 1.23

(4)

Calculation of the Tureover Frequency (TOF). According to the previous reported method,42 the per-site turn frequency (s−1) can be determined by the following equation:

TOF = I /4Fn

(5)

where I is the current on the linear sweep curves (A), F is the Faraday constant (C mol−1), and n is the number of active sites (mol). The factor 1/4 suggested that the formation of one oxygen molecule needs four electrons. The n can be obtained by the total charges for a oneelectron redox process which can be calculated by the absolute area of cyclic voltammograms at a scan rate of 1 mV s−1 in the 1.0 M KOH solution (Figure S13). Overall Water Splitting. To fabricate the full water electrolysis system, the NiS/NF electrode was directly used as both anode and cathode. Before collecting the polarization curves, a continuous N2 flow was purged into electrolyte for at least 30 min. The polarization curves were recorded with a scan rate of 1 mV s−1 in KOH aqueous solution (1.0 M).

Figure 1. (a) Wide-angle XRD pattern, (b−d) SEM, (e) EDS elemental mapping, (f, g) TEM images of NiS/NF. (h, i) Highresolution XPS spectra of the Ni 2p and S 2p core levels for NiS/NF. Inset in (h): XPS survey spectrum of NiS/NF.

X-ray spectroscopy (EDS) elemental mapping images of NiS/ NF (Figure 1e) clearly demonstrate the uniform distribution of Ni and S, which strongly verify the successful chemical conversion of metallic nickel to nickel sulfides via the hydrothermal-sulfurization process. The transmission electron microscopic (TEM) image of the Ni3S4 nanosheets (Figure 1f) exhibits the nanosheet structure. The high-resolution TEM (HR-TEM) image (Figure 1g) presents the visible lattice fringe space of 0.34 nm, corresponding to (220) plane of Ni3S4, being in agreement with the information on XRD pattern. However, decreasing the concentration of Na2S solution from 1.5 to 1.0 M during the preparation process, the obtained sample was named as NiS-10/NF, in which only vertically grow nanosheets on Ni foam was observed (Figure S4) with the phase of Ni3S4 (PDF No. 47−1739, JCPDS, [year]) detected by XRD analysis (Figure S5). While the featureless morphology with several cracks on its skeleton (Figure S6) of the control sample was prepared by direct hydrothermal treatment without the addition of Na2S, named, as Ni/NF, it was drastically different from that of the virtually grown NiS nanosheets, and the XRD analysis (Figure S7) indicates that the Ni/NF also kept the metallic Ni phase without any evident residual peaks. Controlled experiments clearly indicate that the concentrate of Na2S solution is a vital parameter for the fabrication of nanosheet superstructure on Ni foam. The X-ray photoelectron spectroscropy (XPS) was carried out to detect the surface chemistry of NiS/NF. XPS (Figure 1h inset) clearly verifies the presence of Ni and S in NiS/NF, consistent with the above analyzed results. For the highresolution Ni 2p spectrum (Figure 1h), two main peaks at 855.5 and 873.6 eV correspond to Ni 2p3/2 and 2p1/2, respectively, along with two smaller satellites at 864.3 and 882.3 eV. The two subpeaks at 861.1 and 878.8 eV can be ascribed to the oxidized Ni species associated with their main peaks.41,43 Similarly, peak deconvolution of the XPS S 2p presents two peaks at about 161.7 and 163.6 eV, attributed to S 2p3/2 and S 2p1/2, respectively. Additionally, the peak at 167.9

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RESULTS AND DISCUSSION Material Synthesis and Characterization. For the fabrication of NiS/NF, the metallic Ni foam was first immersed into Na2S solution (1.5 M) in autoclave, followed by hydrothermal treatment at 170 °C for 10 h. The high temperature and autogenous pressure under hydrothermal process initialized the effective reaction of metallic Ni and S2+ to yield new species on Ni foam, resulting in the 3D framework of hierarchical porous nanosheets. The optical images (Figure S1) clearly show the change of the surface color of the Ni foam before and after hydrothermal treatment from slivery white to black, indicative of the new moieties successfully grown on Ni foam. The crystallographic information about the NiS/NF was analyzed by XRD as displayed in Figure 1a. Except three evident diffraction peaks assigned to metallic Ni (Powder Diffraction File (PDF) No. 65−2865, Joint Committee on Powder Diffraction Standards (JCPDS), [2004]), all the other diffraction peaks are assigned to Ni3S4 (PDF No. 47−1739, JCPDS, [2004]). As characterized by the scanning electron microscopic (SEM) image shown in Figure 1b, NiS/NF exhibits 3D interconnected open macropores with the size distribution among the range of 200−500 μm, which is similar to that of pristine Ni foam (Figure S2a). Numerous nanoparticles are directly anchored on Ni foam skeleton (Figure 1c), evidently different from the smooth surface of pristine Ni foam (Figure S2b). Furthermore, high-magnification SEM images display abundant virtual nanosheets directly grown on the framework, and some nanosheets stack together to form the nanosphere configuration with a diameter of 3−5 μm (Figure 1d). Noticeably, as shown in Figure S3, vertically aligned sheets with a layer thickness of 20−30 nm are uniformly grown on Ni foam skeleton. The energy dispersive 7205

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linear relationship of its current density against scan rate.5 The CVs from 2 to 10 mV s−1 were recorded in the region of 0.20− 0.25 V for NiS/NF (Figure 2b) and NiS-10/NF (Figure S9). The higher capacitance currents were obtained by the NiS/NF in comparison with those of Ni-10/NF under the same scan rate, implying the large Cdl value of former, namely, higher ECSA. The calculated Cdl of NiS/NiF and NiS-10/NF are about 87 and 73 mF cm−2, respectively, demonstrating that the NiS/NF possesses a larger electrochemically active surface area than NiS-10/NF. This also demonstrates the importance of the tailored configuration for electrochemically active surface area. The nanosheet morphology with stacked nanospheres of NiS/ NF provides more accessible catalytic active sties than that of NiS-10/NF, thus improving its catalytic performance. To study the superior of the superstructure and the integrated electrode configurations on the catalytic activity, the Ni3S4 nanosheets were scraped off from the NiS/NF and recoated on Ni foam with the same loading mass of NiS/NF by the assistance of polymeric binder of Nafion. In this scratched NiS electrode, the ordered nanosheet structure and the 3D porous configuration are partially or totally destroyed. The scratched NiS exhibits much inferior HER activity in comparison to that of NiS/Ni, and it reaches the 10 mA cm−2 at about −187 mV, as shown in Figure S10. The inferior catalytic performance of scratched NiS electrode highlights the important effect of the tailored configuration on electrocatalytic activity. The EIS was employed to study this difference on catalytic activity. For NiS/NF, the small semicircular diameter with increased overpotential indicates a considerable low charge-transfer resistance (RCT). The integrated electrode configuration of NiS nanosheets directly grown on Ni foam effectively enhances electron/charge transfer at the interface of electrodes and electrolyte. Also, the smaller diameter of the semicircle of the NiS/NF suggests transfer impedance of contact and charge smaller than those of scratched NiS (Figure 2c inset). These results suggest that the NiS/NF electrode possesses an enhanced ECSA with a small charge resistance which is responsible for the outstanding HER electrocatalytic activity. Moreover, the directly grown Ni3S4 nanosheets on Ni foam also endow NiS/NF electrode with robust long-term electrochemical stability. Using continuous electrolysis at a certain potential of −0.15 V, the anodic current has negligible loss (6.9%) within 32 h (Figure 2d), while the scratched NiS displays a huge current decay of about 34.6% (Figure S11), implying the robust stability of the self-supported NiS/NF electrode. In addition, after 2000 cycles of CV, the LSV curves have negative attenuation (Figure 2d inset). The structure details of the NiS/NF electrocatalyst after 32 h of HER durability test were characterized by the SEM. The lowmagnification SEM image (Figure S12a) reveals that the postHER NiS/NF still maintains the original 3D hierarchical porous structure, and high-magnification SEM image (Figure S12b) suggests the morphology of nanosheets is similar to that of the fresh NiS/NF (Figure 1b), corroborating its superior robustness for HER electrocatalysis. Oxygen Evolution Reaction. The OER electrocatalytic activity of NiS/NF was also investigated. In consideration of the initial electrochemical oxidation activation process of the transition-metal-based catalysts, all the following dates were collected after scanning for several cycles until stabilization.33,48 The redox peaks at about 1.45 and 1.30 V can be identified (Figure S13), which is attributed to the redox of nickel sulfide

eV was caused by the surface SO42− species due to exposure in air.41 All these characterization results confirm the Ni3S4 nanosheets uniformly grown on conductive Ni foam matrix. Hydrogen Evolution Reaction. The electrocatalytic performance of as-prepared NiS/NF was first studied as HER electrocatalyst in 1.0 M KOH electrolyte under a transitional three-electrode glass cell, as shown in Figure S8. As despised in Figure 2a, the polarization curve of the NiS/NF displays a

Figure 2. (a) Polarization curves and Tafel plots (inset) of Pt/C, NiS/ NF, NiS-10/NF, Ni/NF, and pristine Ni foam (scan rate 1 mV s−1). (b) CVs of NiS/NF measured at different scan rates from 2 to 10 mV s−1. Inset in (b): plot of the current density at 0.23 V versus the scan rate. (c) EIS of NiS/NF recorded at different overpotentials. Inset in (c): equivalent circuit diagram and EIS of NiS/NF and scratched NiS recorded at an overpotential of −320 mV. (d) Chronoamperometric response curves at a constant potential of −0.15 V of NiS/NF. Inset in (d): polarization curves of NiS/NF before and after 2000 CV scans (scan rate: 50 mV s−1). All experiment dates were recoated in 1.0 M KOH solution.

positive onset potential of about 50 mV (vs RHE) and greater anodic current than those of pristine Ni foam, Ni/NF, and NiS10/NF, highlighting the important effect of the well-designed nanostructure. More importantly, NiS/NF can afford 10 mA cm−2 at an overpotential (η) of −122 mV, much better or comparable than those of recently reported non-noble HER catalysts in alkaline medium, such as FeP/CC (5 mA cm−2 at −163 mV),26 NiFe/NiCo2O4/NF (10 mA cm−2 at −105 mV),25 NiCo2O4 microcuboids (10 mA cm−2 at −110 mV),44 VOOH nanospheres (10 mA cm−2 at −164 mV),45 MoS2/ Ni3S2 (10 mA cm−2 at 110 mV),46 and Co3O4-MTA (20 mA cm−2 at −190 mV).47 The detailed comparable information is summarized in Table S1. Furthermore, the smaller Tafel slope (69 mV dec−1) indicates more favorable HER catalytic kinetics on NiS/NF (Figure 1a inset) in alkaline medium, greatly highlighting the superiority of virtual grown Ni3S4 nanosheets. Indeed, the hierarchical nanosheets provide sufficient electroactive sites, as well as facilitate the generated gas release and mass transport, thus benefiting the superior HER activity. The Cdl proportional to the electrochemical double-layer capacitance (ECSA) was used to explore the superiority of tailored configuration. The Cdl value can be calculated from the 7206

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V to afford the cathodic current density of 60 mA cm−2 over 30 h. During the first several hours, the required potential gradually increases. The initial potential enhance is ascribed to the oxidation of Ni3S4 to form active state toward oxygen evolution, greatly referring to the formation of surface oxides/ hydroxyls.5,10,33 Further prolonging the reaction time, the curve remained stable, as depicted by the numerous bubbles release from the electrode surface without any evident aggregation of bubbles to large ones (Figure 3a inset). As shown in Figure 3c, the stable curve is maintained over 20 h to provide 120 mA cm−2 under 1.60 V, further demonstrating the robust stability of NiS/NF even under high operating potential. However, IrO2 exhibits considerable current decay over 20 h due to the fall off of catalysts resulted from the emission of large amount of oxygen, as shown in Figure 3d. The evident difference in stability undoubtedly indicates the superiority of the NiS/NF. The high-magnification SEM image (Figure 4a) shows that the post-OER NiS/NF inherits the overall 3D hierarchical

species.33 The LSV curves about all samples are recorded and shown in Figure 3a. The cathodic current rapidly increases with

Figure 3. (a) Polarization curves. (b) Tafel plots of IrO2, NiS/NF, NiS-10/NF, and pristine Ni foam (scan rate 1 mV s−1). Inset in (a): optical image of NiS/NF directly employed as working electrode at 1.58 V showing the generation of O2 bubbles. (c) Chronopoteniometric curve of the NiS/NF at the current densities of 60 and 120 mV s−1, respectively. (d) Chronopotentiometric response of NiS/NF and IrO2 at a constant current density of 20 mA cm−2 for 20 h. All experiment dates were recorded in O2-saturated 1.0 M KOH solution.

the enhanced potential of IrO2, indicating its drastic OER catalytic activity. It justly needs an overpotential of 315 mV to achieve 20 mA cm−2, while the need for NiS/NF slightly increases to 320 mV to deliver 20 mA cm−2. Additionally, the current density of NiS/NF can surpass on that of IrO2 when the potential higher than 1.56 V (Figure 3a), featuring outstanding electrocatalytic activity of NiS/NF. Moreover, the OER electrocatalytic activity of NiS/NF not only surpasses those of Ni/NF (20 mA cm−2 at 412 mV) and NiS-10/NF (20 mA cm−2 at 344 mV) but also is better than or comparable to that of recent reported nonprecious metal-based catalysts, such as CoCo LDH (10 mA cm−2 at 380 mV),49 NiCo2O4 nanocages (10 mA cm−2 at 340 mV),50 Ni3S2 (10 mA cm−2 at 340 mV),51 and Co(OH)2 (10 mA cm−2 at 320 mV),52 and the detailed comparison is shown in Table S2. The difference of NiS/NF and NiS-10/NF on polarization curves further indicates the importance of morphology. To discuss the importance of morphology, turnover frequencies (TOFs) was also calculated. By integrating the quasireversible oxidation and reduction peaks in the CV curves (Figure S13), the calculated TOF of NiS/NF is 0.013 s−1 (η = 350 mV), slightly higher than the value of NiS-10/NF (0.011 s−1), further demonstrating the positive effect of electrode architecture for electrocatalysis. The Tafel slope of NiS/NF is 71 mV dec−1, slightly higher in comparison to that of IrO2 (55 mV dec−1), as displayed in Figure 3b. The low Tafel slope of NiS/Ni implies the favorable OER kinetic in alkaline solution. Furthermore, NiS/NF also shows robust durability for the OER process. The chronopoteniometric experiment (Figure 3c) reveals that NiS/NF can maintain a stable potential of 1.57

Figure 4. (a) and (b) SEM and SEM-EDS elemental mapping images of post-OER NiS/NF. (c) TEM image of post-OER NiS/NF. (d) XPS of S 2p core level of NiS/NF before and after 20 h OER stability test.

porous configuration which is similar to that of the fresh samples (Figure 1b). The XRD pattern (Figure S14) for the catalyst after long-term OER durability testing reveals that it has no difference in crystal phase in comparison with that of fresh NiS/NF (Figure 1a). The corresponding SEM-EDS elemental analysis is also performed to determine the compositional change after durability test of NiS/NF electrode. As clearly observed in Figure 1e, the Ni and S elements are homogeneously dispersed on Ni foam, indicating the good crystal structure of Ni3S4. However, after long-term OER test, the ratio of O element evidently increase, as displayed in Figure 4b, suggesting some new oxides/hydroxides generated on electrode surface. However, the change of the content toward Ni and S elements are slight in the selected area. The highresolution TEM image (Figure 4c) indicates that the amorphous species surrounds the Ni3S4 materials with evident lattice fringes after long-term OER test, and the corresponding outer species would be related to nickel oxides/hydroxides, directly proved by the enhanced XPS peak intensity assigned to 7207

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ACS Sustainable Chemistry & Engineering oxidized sulfides species of S 2p core level (Figure 4d). However, the high-resolution XPS spectrum of the Ni 2p toward post-OER NiS/NF electrode (Figure S15) reveals the decreased intensity of peaks ascribed to Ni 2p3/2 and 2p1/2 (855.5 and 873.6 eV) and the enhanced intensity at 861.1 and 878.8 eV which correspond to oxidized Ni species. It should be mentioned that such an oxidative transformation toward bifunctional transition metal electrocatalysts has been widely reported.5,6,10,33,38 Similar to our group reporting CoP nanorods OER catalyst,10 it is believed that this in situ formed nickel oxides/hydroxides during OER process of NiS/NF electrode greatly contribute to its superior OER electrocatalytic activity. Overall Water Splitting. The outstanding bifunctional electrocataltic performance toward HER and OER encourage us to construct a water-splitting alkaline electrolyzer assembled by employing NiS/NF as both anode and cathode to investigate its overall water splitting performance due to the impendency of electrocatalysts for the production of hydrogen from electrochemical. In 1.0 M KOH aqueous solution, the NiS/NF couple exhibits high activity toward full water splitting with the required cell voltages of 1.61 and 1.67 V to obtain 10 and 20 mA cm−2, respectively, lower than those values reported with nonprecious metal bifunctional electrocatalysts toward full water splitting, such as CoO−CNF (10 mA cm−2 at 1.63 V),53 Ni2P (10 mA cm−2 at 1.63 V),54 Ni5P4 (10 mA cm−2 at 1.70 V),55 Co3O4-MTA (10 mA cm−2 at 1.63 V),47 NiS nanospheres (10 mA cm−2 at 1.64 V),56 and others summarized in Table S3. The generated gas bubbles release quickly from the electrode surface without evidently aggregating as big bubbles (Figure 5a

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CONCLUSIONS

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ASSOCIATED CONTENT

The novel 3D hierarchical porous Ni3S4 nanosheet frameworks have been obtained through one-pot hydrothermal sulfurization process of metallic Ni foam in Na2S solution, which can be directly employed as bifunctional electrocatalyst without additional polymer binders and substrates toward full water splitting. It justly renders current density of 10 mA cm−2 at overpotential of −122 mV for HER and 320 mV to afford 20 mA cm−2 for OER. Furthermore, as bifunctional electrocatalyst for alkaline electrolyzer toward full water splitting, the NiS/NF couple approaches 10 and 20 mA cm−2 at cell voltages of 1.61 and 1.67 V, respectively, together with superior durability, surpassing most of the nonprecious bifunctional electrocatalysts. Various characterization analysis and experiments demonstrate that the hierarchical interconnected configuration, virtual nanosheet structure, and active constituents together lead to numerous active sites and enhanced mass/charge transport. Moreover, the integrated electrode configuration between active catalyst and conductive metallic Ni foam enable its high activity and robust stability. This synthesis method provides an another pathway to design and fabricate various bifunctional electrodes originated from nonprecious metals and, especially, potentially extends to obtaining other morphologytailored metal sulfides on different metallic collectors for emerging sustainable energy storage and conversion devices.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01419. Optical images of pristine Ni foam and the NiS/NF electrode, SEM images of pristine Ni foam, NiS/NF, NiS-10/NF, XRD pattern of NiS-10/NF, SEM images of Ni/NF electrode and corresponding XRD pattern, optical photographs of electrocatalytic HER process in the three-electrode glass cell, cyclic voltammograms (CVs) of NiS-10/NF measured at different scan rates, polarization curves and corresponding chronoamperormetric response curves of NiS/NF and scratched NiS electrodes, SEM images of post-HER NiS/NF electrode, cyclic voltammograms of NiS/NF and NiS-10/NF, XRD pattern of initial and post-OER NiS/NF electrode, XPS spectrum of Ni 2p for NiS/NF before and after 20 h of OER stability (PDF)

Figure 5. (a) Polarization curves for overall water splitting of a twoelectrode alkaline electrolyzer using NiS/NF|NiS/NF catalyst couple (scan rates 1 mV s−1). Inset in (a): optical image of the electrolyzer generated H2 (left) and O2 (right) at 1.70 V. (b) Chronopotentiometric curve of the NiS/NF catalyst couple at a current density of 10 mA cm−2 for 20 h. Inset in (b): polarization curves of NiS/NF catalyst couple before and after 20 h long-term overall water splitting. All experiment data were recorded in 1.0 M KOH solution.

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AUTHOR INFORMATION

Corresponding Author

inset), indicating the superiority of integrated configuration. Additionally, the NiS/NF couple shows superior long-term durability, as evaluated by the chronopotentiomertic measurement at 10 mA cm−2 (Figure 5b); the applied potential remains at about 1.61 V without evident change during continuous operation of 20 h. The collected polarization curves of NiS/NF couple before and after 20 h long-term stability test (Figure 5b inset) display that there is only a 4.5% anodic current loss, further demonstrating the prominent stable to resist accelerated degradation.

*E-mail: [email protected]. ORCID

Zhong-Yong Yuan: 0000-0002-3790-8181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21421001, 21573115). 7208

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ACS Sustainable Chemistry & Engineering

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